In recent years, the increasing density of microelectronic devices on integrated circuits has lead to a technological bottleneck in the density of metallic signal lines that can be used to interconnect these devices. In addition, the use of metallic signal lines yields a significant increase in power consumption and difficulties with synchronizing the longest links positioned on top of most circuits. Rather than transmitting information as electrical signals via signal lines, the same information can be encoded in electromagnetic radiation (“ER”) and transmitted via waveguides, such as optical fibers, ridge waveguides, and photonic crystal waveguides. Transmitting information encoded in ER via waveguides has a number of advantages over transmitting electrical signals via signal lines. First, degradation or loss is much less for ER transmitted via waveguides than for electrical signals transmitted via signal lines. Second, waveguides can be fabricated to support a much higher bandwidth than signal lines. For example, a single Cu or Al wire can only transmit a single electrical signal, while a single optical fiber can be configured to transmit about 100 or more differently encoded ER.
Recently, advances in materials science and semiconductor fabrication techniques have made it possible to develop photonic devices that can be integrated with electronic devices, such as CMOS circuits, to form photonic integrated circuits (“PICs”). The term “photonic” refers to devices that can operate with either classically characterized ER or quantized ER with frequencies that span the electromagnetic spectrum. PICs are the photonic equivalent of electronic integrated circuits and may be implemented on a wafer of semiconductor material. In order to effectively implement PICs, passive and active photonic components are needed. Waveguides and attenuators are examples of passive photonic components that can typically be fabricated using conventional epitaxial and lithographic methods and may be used to direct the propagation of ER between microelectronic devices. However, these fabrication methods often produce defects in the photonic components that can result in significant channel loss. One common source of loss is scattering due to surface roughness.
FIG. 1 shows a top view of an example microdisk 102. In general, because a microdisk has a larger index of refraction than its surroundings, channels become trapped as a result of total internal reflection near the circumference of the microdisk and may be trapped within the microdisk. Modes of ER trapped near the circumference of the microdisk are called “whispering gallery modes (‘WGMs’).” A directional arrow 104 located near the circumference of microdisk 102 represents a hypothetical WGM propagating near the circumference of microdisk 102. Intensity plot 106 shows intensity of the WGM versus distance along line A-A of microdisk 102. Dashed-line intensity curves 108 and 110 show the WGM confined substantially to a peripheral region of microdisk 102. Portions of curves 108 and 110 that extend beyond the diameter of microdisk 102 represent evanescence of the WGM along the circumference of microdisk 102. However, enlargement 112 of an edge of microdisk 102 shows surface roughness which can be caused by an etching process used to form microdisk 102. This surface roughness increases scattering loss and reduces the Q factor of microdisk 102. Physicists and engineers have recognized a need for photonic components designs and fabrication methods that reduce scattering losses and increase Q factors associated with photonic components.